Message-ID: <D8OPMRYE0SO5.2JQD6ZIYXHP68@proton.me>
Date: Mon, 24 Mar 2025 18:32:53 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Danilo Krummrich <dakr@...nel.org>
Cc: Greg KH <gregkh@...uxfoundation.org>, bhelgaas@...gle.com, rafael@...nel.org, ojeda@...nel.org, alex.gaynor@...il.com, boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com, a.hindborg@...nel.org, aliceryhl@...gle.com, tmgross@...ch.edu, linux-pci@...r.kernel.org, rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/3] rust: pci: impl TryFrom<&Device> for &pci::Device

On Mon Mar 24, 2025 at 7:13 PM CET, Danilo Krummrich wrote:
> On Mon, Mar 24, 2025 at 05:36:45PM +0000, Benno Lossin wrote:
>> On Mon Mar 24, 2025 at 5:49 PM CET, Danilo Krummrich wrote:
>> > On Mon, Mar 24, 2025 at 04:39:25PM +0000, Benno Lossin wrote:
>> >> On Sun Mar 23, 2025 at 11:10 PM CET, Danilo Krummrich wrote:
>> >> > On Sat, Mar 22, 2025 at 11:10:57AM +0100, Danilo Krummrich wrote:
>> >> >> On Fri, Mar 21, 2025 at 08:25:07PM -0700, Greg KH wrote:
>> >> >> > Along these lines, if you can convince me that this is something that we
>> >> >> > really should be doing, in that we should always be checking every time
>> >> >> > someone would want to call to_pci_dev(), that the return value is
>> >> >> > checked, then why don't we also do this in C if it's going to be
>> >> >> > something to assure people it is going to be correct?  I don't want to
>> >> >> > see the rust and C sides get "out of sync" here for things that can be
>> >> >> > kept in sync, as that reduces the mental load of all of us as we travers
>> >> >> > across the boundry for the next 20+ years.
>> >> >> 
>> >> >> I think in this case it is good when the C and Rust side get a bit
>> >> >> "out of sync":
>> >> >
>> >> > A bit more clarification on this:
>> >> >
>> >> > What I want to say with this is, since we can cover a lot of the common cases
>> >> > through abstractions and the type system, we're left with the not so common
>> >> > ones, where the "upcasts" are not made in the context of common and well
>> >> > established patterns, but, for instance, depend on the semantics of the driver;
>> >> > those should not be unsafe IMHO.
>> >> 
>> >> I don't think that we should use `TryFrom` for stuff that should only be
>> >> used seldomly. A function that we can document properly is a much better
>> >> fit, since we can point users to the "correct" API.
>> >
>> > Most of the cases where drivers would do this conversion should be covered by
>> > the abstraction to already provide that actual bus specific device, rather than
>> > a generic one or some priv pointer, etc.
>> >
>> > So, the point is that the APIs we design won't leave drivers with a reason to
>> > make this conversion in the first place. For the cases where they have to
>> > (which should be rare), it's the right thing to do. There is not an alternative
>> > API to point to.
>> 
>> Yes, but for such a case, I wouldn't want to use `TryFrom`, since that
>> trait to me is a sign of a canonical way to convert a value.
>
> Well, it is the canonical way to convert, it's just that by the design of other
> abstractions drivers should very rarely get in the situation of needing it in
> the first place.

I'd still prefer it though, since one can spot a

    let dev = CustomDevice::checked_from(dev)?

much better in review than the `try_from` conversion. It also prevents
one from giving it to a generic interface expecting the `TryFrom` trait.

---
Cheers,
Benno

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